Worldwide Effects of Nuclear War
Note 5: Oxygen, Ozone and Ultraviolet Radiation
Oxygen, vital to breathing creatures, constitutes about one-fifth of the earth's atmosphere. It occasionally occurs as a single atom in the atmosphere at high temperature, but it usually combines with a second oxygen atom to form molecular oxygen (O2). The oxygen in the air we breathe consists primarily of this stable form. Oxygen has also a third chemical form in which three oxygen atoms are bound together in a single molecule (O3), called ozone. Though less stable and far more rare than O2, and principally confined to upper levels of the stratosphere, both molecular oxygen and ozone play a vital role in shielding the earth from harmful components of solar radiation.
Most harmful radiation is in the "ultraviolet" region of the solar spectrum, invisible to the eye at short wavelengths (under 3,000 Å). (An angstrom unit--Å--is an exceedingly short unit of length--10 billionths of a centimeter, or about 4 billionths of an inch.) Unlike X-rays, ultraviolet photons are not "hard" enough to ionize atoms, but pack enough energy to break down the chemical bonds of molecules in living cells and produce a variety of biological and genetic abnormalities, including tumors and cancers.
Fortunately, because of the earth's atmosphere, only a trace of this dangerous ultraviolet radiation actually reaches the earth. By the time sunlight reaches the top of the stratosphere, at about 30 miles altitude, almost all the radiation shorter than 1,900 Å has been absorbed by molecules of nitrogen and oxygen. Within the stratosphere itself, molecular oxygen (02) absorbs the longer wavelengths of ultraviolet, up to 2,420 Å; and ozone (O3) is formed as a result of this absorption process.
It is this ozone then which absorbs almost all of the remaining ultraviolet wavelengths up to about 3,000 Å, so that almost all of the dangerous solar radiation is cut off before it reaches the earth's surface.